7.1 Pediatric Invasive Mechanical Ventilation: Modes, Triggering, Cycling & Waveforms

Pediatric invasive mechanical ventilation

Choose settings based on the child's respiratory mechanics, gas exchange and circulation. Check delivered tidal volume and pressure after changing settings or modes. Read the ventilator graphics with the examination, the airway and the circuit.

Pressure, volume and flow

In a simplified single-compartment model, the airway pressure delivered by the ventilator plus the pressure generated by inspiratory muscle effort balances the elastic and resistive loads above total end-expiratory pressure.

$P_{aw}+P_{mus}=\frac{V}{C_{rs}}+R_{rs}\dot V+PEEP_{tot}$

Here $V$ is volume above the end-expiratory starting volume, $\dot V$ is flow, $C_{rs}$ is respiratory system compliance, and $R_{rs}$ is effective resistance. Use liters and liters per second with compliance in L/cm H₂O and resistance in cm H₂O/(L/s). This approximation assumes linear mechanics and neglects inertance. The lung and chest wall both contribute to compliance. Small tubes, secretions, kinking and bronchospasm can increase the resistive load. Mechanical ventilation taxonomy and equation of motion · Pediatric respiratory mechanics measurements

Worked example: With tidal volume 120 mL, peak pressure 28, plateau 22, set PEEP 6 and total PEEP 8 cm H₂O, intrinsic PEEP is 2 cm H₂O. Static driving pressure is 14 cm H₂O, and static compliance is $120/14\approx8.6$ mL/cm H₂O. With constant inspiratory flow 0.25 L/s, estimated inspiratory resistance is $(28-22)/0.25=24$ cm H₂O/(L/s). AARC patient–ventilator assessment · Pediatric PARDS pressure study

Expiratory time constant

In a passive single-compartment model, the time constant is $\tau=R\times C$. The proportion of the initial volume change completed by time $t$ is $1-e^{-t/\tau}$. At one, two, three and five time constants this is approximately 63.2%, 86.5%, 95.0% and 99.3%. Convert mL to L before multiplying by resistance expressed per L/s.

The model assumes a single resistance and compliance. Children with obstruction may have uneven regional time constants and different inspiratory and expiratory resistance. A calculation using an inspiratory resistance estimate may understate the time needed to exhale. Persistent flow at the next inspiration suggests incomplete emptying. Check for leaks and effort, measure total PEEP when feasible and reassess after allowing more expiratory time. Clinical study of expiratory time constants · Infant bronchiolitis mechanics study

Mode and breath timing

For PARDS, PALICC-2 suggests 6–8 mL/kg of the lesser of predicted and actual body weight. Use 4–6 mL/kg when needed for pressure limitation, with caution below 4 mL/kg. In most children with PARDS the suggested plateau limit is 28 cm H₂O and static driving pressure limit is 15 cm H₂O. Reduced chest-wall compliance may permit a higher plateau under the guideline. For an actual weight of 32 kg and a predicted weight of 24 kg, 6 mL/kg is 144 mL. Reassess measured mechanics, gas exchange and circulation. Postoperative cardiac physiology may change the tolerance of airway pressure. Getinge Servo-u manual · Pediatric PRVC study · PALICC-2 · AAP neonatal cardiac respiratory care

Triggering starts inspiration. A ventilator can trigger based on time, or detect an inspiratory pressure drop or flow change from the child. If breaths occur without effort, inspect the circuit for condensate, check for leaks and compare pressure or flow deflections with the cardiac rhythm. Cardiac oscillations can trigger breaths in pressure and flow modes. Adjust sensitivity or trigger method as appropriate for the device and verify that true patient efforts still trigger breaths.

Cycling ends inspiration. Pressure-controlled mandatory breaths usually cycle by time. Pressure support usually cycles when inspiratory flow falls to a set fraction of peak. For example, at a peak of 20 L/min, a 25% threshold is 5 L/min. A higher percentage usually cycles earlier, which can help when expiration starts before the machine cycles. A lower percentage usually cycles later. Interpret the setting with the patient's effort, leak and expiratory time. Ventilator taxonomy · Manufacturer controls · Cardiac oscillation studies · Trigger bench study

Reading graphics at the bedside

• Flow–time: Expiratory flow continuing into inspiration suggests trapping. Confirm the cause and assess expiratory time, measured PEEP and hemodynamics. A sawtooth tracing can accompany secretions. Also check for circuit water and confirm the clinical indication for suction.
• Pressure–time: During constant-flow volume control, an inspiratory dip with visible effort can suggest unmet flow demand. Check settings, respiratory drive and discomfort. A single contour can have more than one explanation.
• Pressure–volume loop: End-inspiratory flattening, with pressure on the horizontal axis, suggests decreasing compliance and possible overdistension. Compare tidal volume, plateau pressure and driving pressure. The lower inspiratory inflection does not establish the pressure that prevents collapse on deflation. In PARDS, adjust PEEP using oxygenation, oxygen delivery, compliance and hemodynamics rather than adding a fixed amount to a loop landmark. PALICC-2 monitoring · AARC suction guideline · Pressure–volume and CT study